The present invention relates to a thermoelectric module in which junction reliability is enhanced between a thermoelectric element and an electrode and to a method of manufacturing the thermoelectric module.
Thermoelectric modules in which thermal energy is converted into electric energy by use of the Zeebeck effect have advantages, for example, they include no driving section, they are simple in structure, and they are maintenance-free devices. Heretofore, due to low energy conversion efficiency, these modules have been employed in particular apparatuses in limited fields, for example, in power sources for use in the space. However, for the realization of an eco-friendly society, the thermoelectric modules have attracted attention in association with a method to recover thermal energy from exhaust heat. Discussion is underway to apply the thermoelectric modules to incinerators, industrial furnaces, apparatuses associated with cars, and the like. In consideration of this background, it has been desired to enhance durability and conversion efficiency and to lower the cost of the thermoelectric modules.
However, the thermoelectric modules put to practices today mainly include bismuth-tellurium-based thermoelectric elements as described in JP-A-9-293906, and are to be employed at a low temperature of 300° C. or less. Hence, when applying the thermoelectric modules to the industrial furnaces and the cars as described above, it is required to employ thermoelectric elements operable at higher temperature than bismuth-tellurium-based thermoelectric elements, for example, silicon-germanium-based, magnesium-silicide-based, and manganese-silicide-based thermoelectric elements.
In operation of a thermoelectric element, it is possible, by producing a temperature difference in thermoelectric elements, to convert heat into electricity. Hence, in a junction between a thermoelectric element and an electrode, stress takes place due to the thermal expansion difference between the thermoelectric element and the electrode in the module operating environment, leading to a fear of breaking in the junction and the thermoelectric element. The stress increases as the temperature in the operation environment becomes higher; or, as the linear expansion coefficient difference between the thermoelectric element and the material of the junction and that between the thermoelectric element and the electrode become greater. Particularly, this is quite an important phenomenon for a thermoelectric module which may be used in an environment at temperature of 300° C. or more. Further, depending on the allocating location of the thermoelectric module, vibration and shock may take place on the module. When vibration and shock take place in addition to the thermal stress appearing in the module, it is feared that breaking easily occurs in the junction and the thermoelectric element.
JP-A-2000-188429 describes a line-type thermoelectric conversion module in which p-type and n-type thermoelectric elements are connected to an Mo electrode by use of carbon and Ni brazing, and the respective thermoelectric elements are joined via oxide glass with each other, to thereby moderate thermal stress occurring due to temperature difference.
JP-A-2001-189497 describes a thermoelectric conversion element in which a substantially L-shaped n-type semiconductor material and a substantially L-shaped p-type semiconductor material are combined with each other according to the required number thereof and then are hot or cool compression-molded or are subjected to hot or cold processing based on powder metallurgy by use of different metallic materials on the high-temperature and low-temperature sides, into one block by forming pn junctions, the thermoelectric conversion element including a plurality of pn junctions in the connecting direction.
JP-A-2007-109942 describes a thermoelectric module in which p-type and n-type thermoelectric elements or thermoelements having mutually different linear expansion coefficients are alternately arranged on an insulative ceramic substrate, and respective thermoelements and the electrode are joined with each other by stress relaxation layers having mutually different linear expansion coefficients.